1. Field of the Invention
The present invention relates to an environment difference detector for detecting an environment difference.
2. Description of the Related Art
Heretofore, various environment difference detectors for detecting environment differences have been known.
For example, to detect a difference of various gas components, which is an environment difference in an atmosphere, a vapor-phase chemical process or the like, various gas sensors such as a contact combustion type sensor, a semiconductor sensor, and an elastic surface wave sensor have conventionally been used as environment difference detectors. The conventional elastic surface wave sensor uses a planar elastic surface wave element, and thus it is generally said to be high in sensitivity.
For example, the planar elastic surface wave element used for the conventional elastic surface wave sensor includes a substrate made of a piezoelectric crystal such as rock crystal, lithium niobate (LiNbO3), lithium tantalite (LiTaO3), or the like, or a multilayered substrate in which an oxide film is formed on a silicon substrate or a glass substrate, and a piezoelectric thin film made of zinc oxide or aluminum nitride, and the like is further formed on the oxide film.
At two positions on a surface of the substrate separated from each other by a predetermined distance, two bamboo blind-shaped electrodes are arranged which function as a surface acoustic wave exciting unit and a surface acoustic wave receiving unit. Each of the two bamboo blind-shaped electrodes is made of a highly conductive metal such as aluminum or gold. The bamboo blind-shaped electrode as the surface acoustic wave exciting unit subjects a high-frequency signal supplied from a high frequency generating unit to piezoelectric conversion, and excites a surface acoustic wave on the surface of the substrate to propagate it on the surface. Then, the bamboo blind-shaped electrode as the surface acoustic wave receiving unit converts the surface acoustic wave excited and propagated on the surface of the substrate by the bamboo blind-shaped electrode as the surface acoustic wave exciting unit into a high-frequency signal again by piezoelectric conversion to supply it to a detection/output unit.
On the surface of the substrate, a sensitive film that reacts to a specific atom or molecule is additionally disposed between the two bamboo blind-shaped electrodes. This reaction includes for example adsorption or occlusion of the specific atom or molecule, and heat generation with respect to the specific atom or molecule. The sensitive film changes physical properties such as a propagation speed, an attenuation coefficient, a dispersed state, or the like of the elastic surface wave propagated between the two bamboo blind-shaped electrodes, in accordance with a degree of the reaction of the sensitive film to the specific atom or molecule. Accordingly, by measuring the above physical properties, it is possible to evaluate the degree of the reaction of the sensitive film to the specific atom or molecule, and furthermore a concentration of specific atoms or molecules in an environment adjacent to the sensitive film.
In the conventional planar elastic surface wave element configured in the above manner, as the elastic surface wave is diffused in a direction orthogonal to its propagating direction while it is propagated on the surface of the substrate, and there is a limit to a size of the substrate, a surface acoustic wave propagating distance to be set between the two bamboo blind-shaped electrodes is equal to or less than 10 mm. In order to detect an environment difference by using the conventional planar elastic surface wave element which can set only such a short surface acoustic wave propagating distance, a thickness of the sensitive film disposed between the two bamboo blind-shaped electrodes must be set to a certain size or more, e.g., 100 nm or more. However, when the thickness of the sensitive film becomes larger, an environment difference detection speed in the environment difference detector which uses the conventional planar elastic surface wave element becomes slow or the sensitive film is easily damaged.
The present invention has been made under the foregoing situation, and an object of the invention is to provide an environment difference detector which has a simple configuration, which is difficult to fail, which is low in manufacturing cost, and which can measures a desired environment difference quickly and highly accurately.